Coronary artery disease is a leading cause of death in industrialized countries. In the United States, 50-60% of heart attacks occur in people without documented coronary artery disease. A chief contributor to the pathology of the disease is the formation of atherosclerotic plaques. Atherosclerotic plaques are thickened areas in vessel walls which result from an accumulation of cholesterol, proliferating smooth muscle cells, and inflammatory cells.
Atherosclerotic Plaques
In general, an atherosclerotic plaque consists of a raised focal point within the intima having a central core of extra-cellular lipids covered by a fibrous cap. The core within the plaque contains crystalline cholesterol, cholesterol esters, phospholipids, cellular degradation products and collagen remnants. The fibrous cap separates the core of the plaque from the lumen of the blood vessel or artery and is comprised mainly of connective tissues that are a dense, fibrous extracellular matrix made up of collagens, elastins, proteoglycans and other extracellular matrix materials. The fibrous cap varies in thickness, number of smooth muscle cells and macrophages, and collagen content. (Vallabhajosula et aI., 1997, J. Nucl. Med. 38(11): 1788-1796).
Atherosclerotic plaques can be characterized as active and prone to rupture (“vulnerable or high-risk plaques”) or inactive and relatively stable (“stable plaques”). A vulnerable, high-risk or rupture-prone plaque is characterized by an abundance of inflammatory cells (such as macrophages), a thin fibrous cap, and a large lipid core. The size of the lipid pool within the atherosclerotic plaque and the thickness of the overlying fibrous cap are important characteristics predicting the stability of the plaque. The edge of the fibrous cap (the shoulder region) is a location of high stress and predisposed to rupture, in part, due to the accumulation of inflammatory cells (such as macrophages) in the area and their secretion of enzymes that cause degradation of the material that makes up the fibrous cap (Moreno et al, Circulation., 1994, 90:775-8; van der Wal et al., 1994, Circulation 89:36-44.; Jander et al, 1998, Stroke, 29:1625-1630) which can lead to rupture of the plaque.
Rupture of the lipid-laden plaque exposes the highly thrombogenic core and the sub-endothelial vascular smooth muscle cell component of the arterial wall to circulating blood. Platelet activation, adhesion and aggregation follow this almost immediately. Platelet adhesion and activation results in the release of coagulation factors and the initiation of the coagulation cascade. The released growth factors, specifically platelet-derived growth factor (PDGF) stimulate the proliferation and migration of vascular smooth muscle cells. Proliferation and migration of vascular smooth muscle cells can lead to plaque remodeling and increased vascular stenosis, or interact with the platelets leading to enhanced thrombogenesis (Pasterkamp et al., 2000, J. Clin. Basic Cardiol. 3:81-86). The resulting thrombosis caused by the vulnerable plaque can cause unstable angina, acute myocardial infarction, stroke, acute deterioration in peripheral artery disease, or sudden coronary death.
Unstable Angina
The heart requires oxygen-rich blood to function. The right and left coronary arteries branch from the aorta and carry oxygenated blood to the tissues of the heart. When the coronary arteries fail to deliver an adequate amount of oxygen-rich blood (a condition called hypoxia) to the heart, chest pain, pressure, or discomfort, commonly known as angina, result. If this situation is prolonged, oxygen depravation can damage the heart muscle itself (a situation known as ischemia) either reversibly or irreversibly.
Angina is classified broadly as stable or unstable, depending on its severity and pattern of occurrence. Stable angina occurs when increased physical activity (e.g., hurrying across a street or climbing a long flight of stairs) raises the demand for oxygen-rich blood. Due to a possible multitude of factors (the most common of which is one or more occluded coronary arteries), the supply created by the coronary blood flow cannot meet this increased demand and hypoxia results. Unstable angina is understood as anginal pain that occurs with lesser degrees of exertion, increasing frequency, or at rest (i.e., without exertion). Unstable angina that occurs at rest represents the condition in its most serious form. It usually is caused by the formation of a blood clot in a coronary artery at the site of a ruptured plaque and, if left untreated, it may result in a heart attack and irreversible damage to the heart.
Unstable angina is likely due to the partial rupture of a vulnerable plaque that has become unstable. The plaque's partial rupture causes a thrombus to develop, but does not completely occlude the artery. Endogenous clot-fighting mechanisms serve to break up the clot but, over time, the plaque continues to rupture and the clotting episodes repeat. Although this patient may not have yet suffered a myocardial infarction, he or she is at high risk of doing so (e.g., if the unstable plaque completely ruptures or if the endogenous clot fighting mechanisms cannot eliminate the clot before total occlusion of the artery). Disrupted fibrous caps taken post mortem from patients with unstable angina are often more heavily infiltrated with macrophages at the plaque rupture site than plaque from cases of stable angina.
Acute Myocardial Infarction
Acute myocardial infarction (“AMI”) refers to a common clinical condition that leads to necrosis of myocardial tissue. This condition is well known in the art and is characterized by the occurrence of pain (in most cases precordial), characteristic electrocardiographic changes, and an increase in plasma levels of intracellular enzymes (such as creatinine phosphokinase and α-hydroxybutyrate dehydrogenase) or cardiac proteins (such as components of the troponin complex, and myoglobin) released by the necrotic cardiac tissue. AMI may be accompanied by hypotension, circulatory failure, pulmonary edema and arrhythmia. In most cases, but not exclusively, AMI results from vascular injury and thrombosis in the coronary vessels, which causes these vessels to become occluded with subsequent impaired blood flow to the jeopardized myocardium (Fuster et al., 1992, New Engl. J. Med., 326:242-310). In most cases, the time of the occlusion of the coronary vessel can be estimated from the medical history, the course of plasma levels of intracellular heart muscle enzymes and electrocardiographic changes.
The initiating event of many myocardial infarctions (heart attacks) is rupture of an atherosclerotic plaque. Such rupture may result in formation of a thrombus or blood clot in the coronary artery which supplies the infarct zone. The infarct zone or area, as it is commonly referred to, is an area of necrosis which results from an obstruction of blood circulation. The thrombus formed is composed of a combination of fibrin and blood cells. The location, degree and duration of the occlusion caused by the clot determine the mass of the infarct zone and the extent of damage. Ultimately, the extent of myocardial damage caused by the coronary occlusion depends upon the “territory” supplied by the affected vessel, the degree of occlusion of the vessel, the amount of blood supplied by collateral vessels to the affected tissue, and the demand for oxygen of the myocardium whose blood supply has suddenly been limited (Pasternak and Braunwald, 1994, Acute Myocardial Infarction. Harrison's Principles of Internal Medicine, 13th Ed., pgs. 1066-77).
Macrophages and the Inflammatory Response
Macrophages are involved in the cause and/or pathology of some coronary syndromes. Macrophage secretion of proteolytic proteins that degrade the fibrous caps of plaques decrease cap thickness as well as increase additional macrophage infiltration thus contributing to plaque instability. Therefore macrophages are considered to have a central role in plaque rupture and their presence in large concentrations is considered predictive to such rupture. Indeed, erosion and/or disruption of the fibrous cap of atherosclerotic plaques is known to modulate arterial thrombus formation, leading to the onset of acute ischemic events. It is clear that rupture at the site of a vulnerable atherosclerotic plaque is the most frequent cause of acute coronary syndromes, such as unstable angina, myocardial infarction or sudden death.
Inflammation has been related both to the pathogenesis of acute myocardial infarctions and to the healing and repair following AMI. Myocardial ischemia prompts an inflammatory response. In addition, reperfusion, the mainstay of current acute therapy of AMI, also enhances inflammation. Reperfusion involves the rapid dissolution of the occluding thrombus and the restoration of blood flow to the area of the heart which has had its blood supply cut off. The presence of inflammatory cells in the ischemic myocardial tissues has traditionally been believed to represent the pathophysiological response to injury. However, experimental studies have shown that while crucial to healing, the influx of inflammatory cells into tissues, specifically macrophages which are phagocytic cells, results in tissue injury beyond that caused by ischemia alone.
Macrophages and other leukocytes infiltrate the myocardium soon after ischemia ensues. Macrophages secrete several cytokines, which stimulate fibroblast proliferation. However, the activated macrophages also secrete cytokines and other mediators that promote myocardial damage. Accordingly, the influx of macrophages into the myocardium increases myocardial necrosis and expands the zone of infarct. Thus, although the acute phase of inflammation is a necessary response for the healing process, persistent activation is in fact harmful to the infarct area as well as the area surrounding it, the so-called ‘peri-infarct zone’.
The inflammatory response that follows myocardial ischemia is critical in determining the severity of the resultant damage caused by the activated macrophages. Plasma levels of inflammatory chemotactic factors (macrophage chemoattractant protein-1 (MCP-1), macrophage inflammatory protein-1 alpha (MIP-1 alpha), have been shown to correlate with subsequent heart failure and left ventricular dysfunction (see, for example, Parissis, et al., 2002, J. Interferon Cytokine Res., 22(2):223-9). Peripheral monocytosis (an elevated number of monocytes) at two and three days after AMI is associated with left ventricular dysfunction and left ventricular aneurysm, suggesting a possible role of monocytes in the development of left ventricular remodeling after reperfused AMI (Mackawa, Y. et al., 2002, J. Am. Coll. Cardiol., 39(2):241-6). Left ventricular remodeling after acute myocardial infarction is the process of infarct expansions followed by progressive left ventricular dilation and is associated with an adverse clinical outcome. Furthermore, plasma levels of macrophage chemoattractant protein-1 (MCP-1) are elevated in patients with acute myocardial infarction. MCP-1 is induced by myocardial ischemia/reperfusion injury and neutralization of this chemokine significantly reduced infarct size.
Suppression of the inflammatory response by nonspecific anti-inflammatory composites after coronary occlusion, in several coronary occlusion/reperfusion models, was shown to reduce the infarct area (See, for example, Squadrito, et al., 1997, Eur. J. Pharmacol.; 335:185-92; Libby, et al., 1973, J. Clin. Invest., 3:599-607; Spath, et al., 1974, Circ. Res., 35: 44-51). However, these nonspecific regimens are associated with adverse effects, such as interference with scar formation and healing, and, in some patients, the development of aneurysm and rupture of the ventricular wall. As such, these regimens are precluded from clinical use. However, animal models that have a decreased ability to suppress macrophage function due to a deficiency in the anti-inflammatory cytokine interleukin-10 were shown to suffer from increased infarct size and myocardial necrosis in a coronary occlusion model (Yang, Z. et al., 2000, Circulation, 101:1019-1026.)
One object of the present invention is the identification of therapeutic agents capable of blocking the accumulation of and/or the biological function including secretion of factors from phagocytic cells (particularly macrophages and monocytes) in the patient suffering from an acute coronary syndrome (particularly unstable angina or and acute myocardial infarction).
Another object of the invention is the development of methods for treating an acute coronary syndrome (particularly unstable angina or and acute myocardial infarction) as well as stabilizing the plaques associated with these syndromes.